The present invention relates to a pulse Doppler measuring apparatus, and more particularly to an apparatus for detecting the velocity of an object by using an ultrasonic wave, for example, a pulse Doppler measuring apparatus capable of measuring the speed of blood flow with a high signal-to-noise ratio, in a case where the blood flow speed in a living body is measured in real time.
Various kinds of apparatuses have hitherto been known which detect the flow speed of an object by utilizing the Doppler effect of an acoustic wave. Specifically, in an apparatus using the pulse Doppler method which is described in, for example, an article entitled "Pulsed Ultrasonic Doppler Blood Flow Sensing" by D.W. Baker (IEEE Trans. SU-17, No. 3, pages 170 to 185, 1970), as is known, it is possible to specify a measured part by transmitting an ultrasonic pulsed continuous wave repeatedly and by setting a time gate corresponding to the distance to the measured part on a reception signal.
As conventional ultrasonic Doppler blood flow measuring apparatuses, as disclosed in, for example, JP-A-58-188433, JP-A-60-119929, and JP-A-61-25527, there are known apparatuses for measuring a blood flow by transmitting an ultrasonic wave toward a blood vessel and by measuring the Doppler shift frequency of the ultrasonic wave reflected from the blood in the blood vessel to measure vcos.theta., where .theta. indicates the angle between the direction of the blood flow and the transmission direction of the ultrasonic wave, and indicates a blood flow speed.
Further, a technique called "color flow mapping" for measuring the distribution of blood flow speed at a certain cross section of a living body to display the distribution on a tomographic image, is described in an article entitled "Real-Time Two-Dimensional Blood Flow Imaging Using an Autocorrelation Technique" by C. KASAI et al., IEEE Trans. SU-32, No. 3, pages 458 to 464, 1985. In order to achieve a desired image frame rate in the color flow mapping, the blood flow speed at each of pixels is determined by averaging the measured values of a plurality of Doppler shift obtained by a relatively small number of measurements. In the example described above, the autocorrelation method is used, in which a difference vector between a vector indicated by a Doppler signal detected currently and a vector indicated by a Doppler signal detected by the preceding measurement is detected by an autocorrelator each time the measurement is repeated, and an average speed is calculated from the argument of a vector given by the sum of a plurality of difference vectors.
Meanwhile, U.S. Pat. No. 4,809,703 discloses the so-called "two axial component method", in which the phase difference .DELTA..theta. of a Doppler signal is detected each time the measurement is repeated, to be decomposed into a cosine component and a sine component, a plurality of values obtained for each of these components are added and averaged, and the phase difference indicated by the average cosine component and the average sine component is transformed into a velocity.
Further, on pages 348 to 352 of 1978 Ultrasonic Symposium Proceedings is described a method, in which the phase difference of a Doppler signal is obtained each time the measurement is repeated, an average phase difference is calculated by adding a plurality of the phase difference values directly, and the average phase difference thus obtained is transformed into a velocity. This method will hereinafter be referred to as "phase difference averaging method".
Meanwhile, it is pointed out by U.S. Pat. No. 4,905,206 that when a true average phase difference is close to +.pi. or -.pi., that is, a high-speed region is measured, the phase difference averaging method produces a large calculation error, and that when the true average phase difference is close to zero, that is, a low-speed region is measured, the autocorrelation method and the two axial component method produce a large calculation error. Further, in this U.S. Patent are described a circuit configuration for changing one of the two methods over to the other so that the above difficulties are eliminated, and a circuit configuration for transforming phase difference values obtained by repeated measurement into phase difference values in a new polar coordinate system having a reference axis which is indicated by the angle of the average phase difference calculated by the autocorrelation method, and for adding and averaging the phase difference values in the new polar coodinate system.